To increase cellular system capacity, one approach that shows promise for substantial capacity enhancement is multiple input and multiple output (MIMO) transmission and reception based on multiple transmit and multiple receive antennas. This has been suggested for use on forward link channels such as the forward-packet-data channel (F-PDCH) in 1xEV-DV [1,2]. To further increase system capacity, Lucent proposed a 2×2 MIMO architecture based on the per-antenna-rate-control (PARC) principle [3]. This scheme has advantages in terms of average high data rates achieved compared to single stream transmission schemes such as those used with minimum-mean square error (MMSE) based dual receive diversity mobile station (MS), phase sweeping transmit diversity (PSTD) and space-time transmit diversity (STTD/STS). One of the main drawbacks with PARC, however, is the high residual frame error rate (FER) compared with the simple dual-receive diversity MS. Thus, it requires a large number of retransmissions to complete a data transmission, particularly in high velocity environments.
STTD is an open loop technique in which the symbols are modulated using the technique of space-time block coding described in [4]. The STTD transmitter and STTD receiver are illustrated in FIG. 1. FIG. 1 shows two symbols S1, S2 entering an STTD encoder 10. This produces at its output a STTD signal to be transmitted over to transmit antennas 12,14. During the first transmit period, w1S1 is transmitted on the first antenna and −w2S2* is transmitted on the second antenna 14. In the following transmit period, w1S2 is transmitted on the first antenna 12, and w2S1* is transmitted on the second antenna 14. It can be seen that the same information is transmitted twice, both during different times and on different antennas. This is transmitted to receive antennas 16 and 18. There is a respective channel between each of the transmit antennas 12,14 and each of the receive antennas 16,18. The four channels are shown as h11, h12, h21, and h22. The signals received by the two receive antennas 16,18 are passed to an STTD decoder 20 which recovers received versions S2˜, S1˜ of the transmitted symbols S1,S2. In FIG. 1, the expression w1S1 represents a Walsh space multiplied by a transmit packet. More particularly, if a Walsh space is available consisting of M Walsh codes is available, this Walsh space is divided among the two antennas, and the contents of the packet divided into segments for transmission using each Walsh code. Thus, in a 16 Walsh code Walsh space, 8 Walsh codes would be transmitted on each transmit antenna, and the transmit packet would be divided into 8 segments with one segment being transmitted using each Walsh code. Each symbol might be an MPSK symbol for example or any other suitable modulation symbol. However, for space-time encoding it is necessary that the symbols lend themselves to complex conjugation.
The STTD scheme is particularly simple. It implements the space-time block code (2×2 code matrices). The orthogonality property of the code matrices allows the symbols from the two transmit antennas to be separated at the receiver. Thus, it may achieve a significant coding gain on space and time as opposed to other diversity schemes such as orthogonal transmit diversity (OTD).
The PARC MIMO system is illustrated in FIG. 2. In this example, the data sequence can be seen to be demultiplexed into two separate streams 30,32. Each stream is assigned an independent modulation/coding/rate. Each of the two streams is used in BLAST encoder 31 to generate a separate Walsh code spread signal that is transmitted from only one of the antennas. In this case, the entire available Walsh space is assigned on both antennas, and as such there is no Walsh code splitting. This is illustrated by showing each symbol being multiplied by the combination of w1+w2. This means that the packet is divided into a number of segments equal to the number of Walsh codes and an equal portion of the packet is transmitted using each Walsh code. For example, if there are 16 Walsh codes in the available Walsh space, the packet would be divided into 16 portions and each portion transmitted using a respective Walsh code. It can be seen that there is no space diversity at the transmitter or time diversity at the transmitter. However, at the receiver there is space diversity since the signal is received at two different antennas.
This design is capable of increasing the system capacity by a factor of two as opposed to 1xEV-DV system.
It should be noted that the transmit encoder packets may belong to either the same user or different users, resulting in different requirements for control signaling. In case of the same user served by two transmit antennas, this requires all encoder packets to occupy the same number of transmission slots so that the transmission for the packets can be always started at the same time. This requires two feedback signal for transmit packet length but requires two feedback signals for packet rate control due to the closed loop MIMO scheme. In case of the different users served by different transmit antennas, this does not require encoder packets to occupy the same number of transmission slots, but requires twice the feedback information for transmit packet length.
Even when the encoder packets belong to the same user, this system still requires two feedback channels (packet data control channel, PDCCH). This is because the channel feed back signal includes the number of packets, modulation and code rate information (in this case, the modulation and code rate, in general, are not the same on different antenna due to the difference reported CIR), and it is not possible to employ only one channel to complete such information feedback.